There is known a writing material and a printing apparatus using decoloration ink. The decoloration ink is decolored by heat, light or chemical processing. Accordingly, characters and figures described or printed with the decoloration ink are decolored by heat, light or chemical processing, and are visually erased.
There are documents where the decoloration ink is unsuitably used such as a deed, a ledger sheet and a medical record. For tamper-proof, it is desirable that the description with the decoloration ink on the above-described documents be detected at the early stage. It may be desirable that a person could recognize the use of the decoloration ink only by seeing. However, in many cases, it is difficult to identify the use of the decoloration ink only by a visual observation.